Cases of swine flu in the UK have almost doubled this week, the Department of Health revealed today, as the outbreak imposes increasing pressure on intensive care beds.

So far 122 people have died after contracting the H1N1 virus and more than 500 are being treated in hospital, of whom 99 are in critical care – the highest figure since the disease emerged. Children appear to be vulnerable to higher rates of infection.

Emergency planners have, however, downgraded the UK's worst case scenario, calculating this month that as many as 1,000 people could die during the pandemic infection. Far more of the victims are likely to be younger patients, unlike the normal pattern of seasonal flu, which affects the elderly most severely.

Estimates of likely casualties have come down progressively over the past few months from an initial upper assessment of 65,000 deaths, later reduced to 19,000 and now cut back again.

The government's emergency contingency committee, Cobra, met this morning to consider the disease's progress. The number of cases has climbed over the past week from 27,000 to 53,000.

Following the deaths of two children at the same special needs school in Northern Ireland in the space of a week, those in special schools may be given extra priority for vaccination.

Revised guidance for NHS planners, released today, suggests that as many as 35,000 people could require hospital treatment during the outbreak and as much as 15% of that number may need critical care treatment.

In the peak week of the outbreak, expected sometime this winter, up to 1.5 million people could be suffering from swine flu, the new guidance suggests. A third of the UK's children could catch the virus, whereas the "clinical attack rate" for the rest of the population is expected to be around 12%. Infection rates are higher in northern England at present.

"We are starting to see the disease creep [southwards] down the country," explained Sir Liam Donaldson, the chief medical officer. "We don't understand why so many [are in critical care now]."

In one NHS region, the East Midlands, almost 17% of paediatric critical care beds are occupied by children suffering from swine flu.

The government's Medicines and Healthcare products Regulatory Agency (MHRA) yesterday promised to monitor any reports of side-effects among those vaccinated. The agency is opening up a website – www.mhra.gov.uk/swineflu – for anyone inoculated to register adverse reactions.

In Northern Ireland, Dr Michael McBride, the province's chief medical officer, warned: "It is going to be a long, difficult winter. It could be one of the most difficult winters we have seen."